The present invention relates to nonaqueous electrolyte secondary cells, such as cylindrical lithium ion secondary cells, which comprise an electrode unit encased in a battery can and serving as an electricity generating element and which are adapted to deliver the electricity generated by the electrode unit to the outside via a positive terminal portion and a negative terminal portion like. The invention relates also to processes for fabricating such cells.
Nonaqueous electrolyte secondary cells of the type mentioned comprise a rolled-up electrode unit formed by laying a positive electrode and a negative electrode, each in the form of a strip, over each other in layers with a separator interposed therebetween and rolling up the resulting assembly into a spiral form. The rolled-up electrode unit is encased in a battery can.
The electric power generated by the rolled-up electrode unit is delivered to the outside through an arrangement including a plurality of conductive current collector tabs having their base ends attached to each of the positive electrode and the negative electrode of the electrode unit. The positive current collector tabs extending from the positive electrode have outer ends connected to a positive terminal portion, and the negative current collector tabs extending from the negative electrode have outer ends connected to a negative terminal portion. This arrangement is widely used.
However, the current collecting arrangement comprising a plurality of collector tabs has the problem of failing to achieve a sufficient current collecting effect when used in nonaqueous electrolyte secondary cells of large size having a high current value since the cell has increased electrode areas although producing a satisfactory current collecting effect in nonaqueous electrolyte secondary cells of small size which are relatively low in current value.
Further the connection of the current collector tabs to each electrode terminal portion requires a complex structure and complicated procedure, hence the problem of low work efficiency or productivity.
Accordingly, a cylindrical nonaqueous electrolyte secondary cell has been proposed which has a current collecting structure comprising a negative electrode current collector plate 36 and a positive electrode current collector plate 30 as shown in FIG. 7. This cell has a battery can 1 formed by a cylinder 15 and lids 16, 16 secured to opposite open ends of the cylinder. A rolled-up electrode unit 2 is enclosed in the battery can 1. The negative electrode collector plate 36 and the positive electrode collector plate 30 are arranged at respective ends of the electrode unit 2 and joined to the unit 2 by laser welding. The collector plates 36, 30 are connected by lead portions 37, 34 respectively to a negative terminal assembly 4 and a positive terminal assembly 40 mounted on lids 16, 16.
The rolled-up electrode unit 2 comprises a positive electrode 23, separator 22 and negative electrode 21 each in the form of a strip. The positive electrode 23 is formed by coating a current collector of aluminum foil with a positive electrode active material. The negative electrode 21 is formed by coating a current collector of copper foil with a negative electrode active material.
The positive electrode 23 and the negative electrode 21 are each superposed on the separator 22, as displaced from the separator widthwise thereof and rolled up into a spiral form, whereby the edge of the positive electrode 23 is positioned as projected outward beyond the edge of the separator 22 at one of opposite ends of the electrode unit 2 in the direction of its winding axis, and the edge of the negative electrode 21 is positioned as projected outward beyond the edge of the separator 22 at the other end of the unit 2. The positive electrode current collector plate 30 is made of aluminum, and the negative current collector plate 36 is made of copper.
With the current collecting structure wherein the collector plates 36, 30 are joined to the respective ends of the electrode unit 2 as described above, the collector plates can be welded to the unit 2 contactlessly without applying pressure to the plates for welding. This achieves an improved work efficiency or productivity.
The process for fabricating the nonaqueous electrolyte secondary cell shown in FIG. 7, however, has the problem that when the negative electrode collector plate 36 is disposed at and welded to the edge of the negative electrode 21 of the unit 2, sufficient energy can not be given to the portion to be welded since the copper forming the collector plate 36 has high reflectivity for the laser beam used for welding, forming a faulty weld and increasing the electric resistance between the unit 2 and the negative electrode collector plate 36 to result in an impaired current collecting efficiency. If the collector plate 36 is made from nickel, the weldability of the plate 36 to the electrode unit 2 can be improved, whereas the collector plate 36 of nickel has greater electric resistance than the plate 36 of copper and therefore exhibits a lower current collecting efficiency.
FIGS. 20 and 23 show another conventional nonaqueous electrolyte secondary cell, which comprises a cylindrical battery can 1 including a cylinder 15 and lids 16, 16 welded to respective opposite ends of the cylinder, and a rolled-up electrode unit 5 enclosed in the can 1. A pair of positive and negative terminal assemblies 110, 110 are mounted on the respective lids 16, 16 and each connected to the electrode unit 5 by a plurality of electrode tabs 6 for delivering the electric power generated by the unit 5 to the outside through the terminal assemblies 110, 110. Each lid 6 is provided with a gas vent valve 13 which is openable with pressure.
As shown in FIG. 22, the rolled-up electrode unit comprises a positive electrode 51 and a negative electrode 52 each in the form of a strip and rolled up into a spiral form with a striplike separator 52 interposed between the electrodes. The positive electrode 51 is prepared by coating opposite surfaces of a striplike current collector 55 of aluminum foil with a positive electrode active material 54 comprising a lithium containing composite oxides. The negative electrode 53 is prepared by coating opposite surfaces of a striplike current collector 57 of copper foil with a negative electrode active material 56 containing a carbon material. The separator 52 is impregnated with a nonaqueous electrolyte.
The positive electrode 51 has an uncoated portion having no active material 54 applied thereto, and base ends of the electrode tabs 6 are joined to the uncoated portion. Similarly, the negative electrode 53 has an uncoated portion having no active material 56 applied thereto, and base ends of the electrode tabs 6 are joined to the uncoated portion.
With reference to FIG. 23, the electrode tabs 6 of the same polarity have outer ends 61 connected to one electrode terminal assembly 110. For the sake of convenience, FIG. 23 shows only some of the electrode tabs as connected at their outer ends to the terminal assembly 110, with the connection of the other tab outer ends to the assembly 110 omitted from the illustration.
The electrode terminal assembly 110 comprises an electrode terminal 111 extending through and attached to the lid 16 of the battery can 1. The electrode terminal 111 has a base end formed with a flange 112. The hole in the lid 16 for the terminal 111 to extend therethrough has an insulating packing 113 fitted therein to provide electrical insulation and a seal between the lid 16 and fastening members. The terminal 111 has a washer 114 fitted therearound from outside the lid 16, and a first nut 115 and a second nut 116 which are screwed thereon. The insulating packing 113 is clamped between the flange 112 of the terminal 111 and the washer 114 by tightening up the first nut 115 to produce an enhanced sealing effect. The outer ends 61 of the electrode tabs 6 are secured to the flange 112 of the terminal 111 by spot welding or ultrasonic welding.
Lithium ion secondary cells have the problem that an increase in the size thereof lengthens the positive and negative electrodes, consequently lowering the current collecting efficiency of the current collecting structure comprising electrode tabs to produce variations in internal resistance or result in a lower discharge capacity.
FIG. 21 shows a current collecting structure proposed to obtain a uniform current collecting efficiency over the entire lengths of the positive and negative electrodes. The proposed structure is provided for a rolled-up electrode unit 7, which comprises a positive electrode 71 prepared by coating a current collector 75 with a positive electrode active material 74, a negative electrode 73 formed by coating a current collector 77 with a negative electrode active material 76 and a separator 72 impregnated with a nonaqueous electrolyte. The positive electrode 71 and the negative electrode 73 are each superposed on the separator 72 as displaced widthwise of the separator, and rolled up into a spiral form, whereby the edge 78 of current collector 75 of the positive electrode 71 is positioned as projected outward beyond the edge of the separator 72 at one of opposite ends of the electrode unit 7 in the direction of its winding axis, and the edge 78 of current collector 77 of the negative electrode 73 is positioned as projected outward beyond the edge of the separator 72 at the other end of the unit 7.
A disklike current collector plate 62 is secured to each of opposite ends of the rolled-up electrode unit 7 by resistance welding and connected to the same electrode terminal assembly 110 as described above by a lead member 63.
The nonaqueous electrolyte secondary cell with the current collecting structure of FIG. 21, however, has the problem of being great in the internal resistance of the cell because the edges 78, 78 of the current collectors 75, 77 forming the positive electrode 71 and the negative electrode 73 of the electrode unit 7 have a small area, therefore providing a small area of contact between the collector plate 62 and each current collector edge.
It is especially required that lithium ion secondary cells, for example, for use as power sources in electric motor vehicles be of high capacity and reduced in internal resistance to the greatest possible extent so as to obtain a high power. Furthermore a current collecting structure of high productivity is required for a reduction of manufacturing cost.
Accordingly, a cell of low resistance and high productivity has been proposed which comprises a current collector plate having small bulging portions formed thereon as uniformly distributed over the entire surface thereof, such that the collector plate is secured to a current collector edge by resistance welding with the bulging portions in contact therewith to concentrate the current on the bulging portions and give improved weld strength (see, for example, JP-U No. 156365/1980).
As shown in FIG. 24, also proposed is a current collecting structure which comprises a current collector plate 92 prepared by forming a plurality of bent portions 94 on a flat platelike body 93, the bent portions 94 being secured to a current collector edge 78 of a rolled-up electrode unit 7 by resistance welding with the collector plate 92 pressed against the current collector edge 78 (see, for example, JP-A No. 31497/1999).
Further known are a current collector plate comprising two divided segments for suppressing ineffective current involved in attaching the collector plate by resistance welding to achieve an improved welding efficiency (JP-A No. 29564/1995), and a current collector plate having a projection V-shaped in section and formed on the portion thereof to be joined by resistance welding so as to concentrate the welding current on the projection and afford improved weld strength (JP-B No. 8417/1990).
Further proposed is a current collecting structure comprising a current collector member 95 in place of the disklike collector plate and formed with a plurality of slits 96 as seen in FIG. 25. For laser welding, a laser beam is projected onto the surface of the collector member 95 as disposed at an end of a rolled-up electrode unit 7, with a current collector edge 78 fitted in the slits 96 of the member 95 (JP-A No. 261441/1998).
Also proposed is a structure wherein a disklike current collector plate has a plurality of projections, V-shaped in section and up to 90xc2x0 in end angle, and is welded to a group of electrode plates by irradiating the projections with a laser beam, with the collector plate pressed against each current collector (JP-B No. 4102/1990).
However, with the above-mentioned current collecting structure wherein the current collector plate is formed with small bulging portions as uniformly distributed over the entire surface thereof (JP-U No. 156365/1980), the collector plate is in unstable contact with the current collector, and the current fails to flow across these members depending on the state of contact, entailing the problem of producing a faulty weld.
The current collecting structure wherein the current collector plate has projections which are V-shaped in section or bent portions for the resistance welding of the plate (JP-A No. 31497/1999, No. 29564/1995 or JP-B No. 8417/1990) has the problem of low weld strength when the current collector has a very small thickness as is the case with lithium ion secondary cells.
The current collecting structure wherein the current collector member having a plurality of slits is secured to the current collector edge by laser welding (JP-A No. 261441/1998) not only requires the collector member which has a complex shape but also has the problem that the work of inserting the current collector edge into the slits of the collector member is very cumbersome.
With the structure wherein the disklike current collector plate having projections of V-shaped section is joined to the group of electrode plates by laser welding (JP-B No. 4102/1990), the projections have a V-shaped section of acute angle, so that the area of contact between the projection and the current collector edge is small, consequently entailing the problem of increased contact resistance. Since the junction between the V-shaped projection and the current collector edge is at an acute angle with the direction of projection of the laser beam for irradiating the junction, the laser beam fails to act effectively to weld the junction and is likely to produce a faulty weld.
A first object of the present invention is to provide the construction of a nonaqueous electrolyte secondary cell having a current collecting structure wherein a negative electrode current collector plate is secured to an end of an electrode unit by welding, and to provide a process for fabricating the cell, the collector plate having improved weldability to the electrode unit.
A second object of the invention is to provide a nonaqueous electrolyte secondary cell having a current collecting structure which is high in productivity and which is so adapted that even when a current collector forming an electrode unit is very thin, an edge of the current collector can be joined to a current collector plate over an increased area of contact, and a process for fabricating the cell.
Construction for Fulfilling First Object
The present invention provides a nonaqueous electrolyte secondary cell comprising an electrode unit 2 which includes a negative electrode 21 having a projecting edge at one of opposite ends of the electrode unit in the direction of winding axis thereof. A negative electrode current collector plate 3 is joined to the edge and electrically connected to a negative terminal portion. The collector plate 3 comprises a plurality of layers including a copper layer 31 made of copper or an alloy consisting predominantly of copper, and a metal layer made of a metal not forming an intermetallic compound with lithium and having a lower laser beam reflectivity than copper or an alloy consisting predominantly of the metal. The copper layer 31 and the metal layer provide opposite surface layers of the collector plate 3, and the copper layer 31 is welded to the edge of the negative electrode 21. The metal for forming the metal layer of the negative electrode current collector plate 3 is, for example, nickel, stainless steel, titanium, chromium or molybdenum.
When the collector plate 3 is welded to the negative electrode edge of the electrode unit 2 with a laser beam in the process for fabricating the nonaqueous electrolyte secondary cell of the invention, the laser beam can be sufficiently absorbed by the collector plate 3 for perfect welding since the laser beam impinging side of the plate 3 is provided by the metal layer which is low in laser beam reflectivity.
The metal layer of the collector plate 3 is made of a metal not forming an intermetallic compound with lithium or an alloy consisting predominantly of the metal and is therefore unlikely to consume lithium ions in the nonaqueous electrolyte to form an alloy, consequently precluding the lithium ion concentration of the nonaqueous electrolyte from reducing.
Further because the negative electrode current collector plate 3 comprises a plurality of layers, i.e., the copper layer 31 and the metal layer, the high conductivity of the copper layer gives the plate 3 lower electric resistance and higher electric conductivity than when the plate 3 consists solely of the metal layer.
The edge of the negative electrode 21 of the electrode unit 2 is joined to the copper layer 31 of the collector plate 3 over the entire length thereof, consequently making it possible to collect the current from the entire electrode unit 2 uniformly even if the cell is large-sized with an increase in the length of the electrodes. This reduces the potential gradient along the length of the negative electrode 21, giving a uniform current distribution, whereby a high current collecting efficiency can be achieved.
Stated more specifically, the negative electrode current collector plate 3 has a thickness in the range of 0.10 mm to 5.00 mm. If the thickness is smaller than 0.10 mm, the collector plate 3 itself has increased electric resistance, which not only results in a lower current collecting efficiency but also permits the collector plate 3 to become melted to excess by laser welding to produce a cave-in in the weld. If the thickness is in excess of 5.00 mm, on the other hand, welding of the collector plate 3 requires increased power, presenting difficulty in welding the collector plate 3 to the negative electrode edge which is tens of micrometers in thickness.
Further stated more specifically, the ratio of the thickness of the metal layer to the thickness of the negative electrode current collector plate 3 is in the range of at least 5% to not greater than 45%. This enables the metal layer to fully serve the function of exhibiting reduced laser beam reflectivity, also permitting the copper layer 31 to satifactorily perform the function of exhibiting reduced electric resistance. If the ratio is smaller than 5%, the metal layer disappears on melting immediately after the start of welding of the collector plate 3 to expose a surface of high laser beam reflectivity, hence impaired weldability. When the ratio is in excess of 45%, on the other hand, the metal layer becomes predominant with respect to the electric resistance of the collector plate 3, increasing the overall electric resistance of the plate 3.
The present invention further provides a process for fabricating a nonaqueous electrolyte secondary cell which process has the steps of:
preparing an electrode unit 2 by laying a positive electrode 23 and a negative electrode 21 over each other with a separator 22 sandwiched therebetween so as to project an edge of the positive electrode 23 at one of opposite ends of the electrode unit 2 and to project an edge of the negative electrode 21 at the other end and rolling up the resulting assembly into a spiral form,
preparing a positive electrode current collector plate 30 from aluminum or an alloy consisting predominantly of aluminum,
preparing a negative electrode current collector plate 3 comprising a plurality of layers including a copper layer 31 made of copper or an alloy consisting predominantly of copper, and a metal layer made of a metal not forming an intermetallic compound with lithium and having a lower laser beam reflectivity than copper or an alloy consisting predominantly of the metal, the copper layer 31 and the metal layer providing respective opposite surface layers of the collector plate 3,
welding the positive electrode current collector plate 30 to the edge of the positive electrode 23 by placing the collector plate 30 at the end of the electrode unit 2 having the projecting edge of the positive electrode 23 and irradiating a surface of the collector plate 30 with a laser beam,
welding the negative electrode current collector plate 3 to the edge of the negative electrode 21 by placing the collector plate 3 at the end of the electrode unit 2 having the projecting edge of the negative electrode 21, with the copper layer 31 in contact with the negative electrode edge, and irradiating a surface of the metal layer of the collector plate 3 with a laser beam, and
assembling a nonaqueous electrolyte secondary cell by electrically connecting the positive electrode current collector plate 30 and the negative electrode current collector plate 3 which are welded to the electrode unit 2 to a positive terminal portion and a negative terminal portion respectively.
In the step of welding the negative electrode current collector plate 3 to the edge of the negative electrode 21 with a laser beam in the fabrication process of the invention described above, the laser beam is projected on the surface of the metal layer of low reflectivity, so that the energy of the laser beam can be fully given to the junction of the collector plate 3 and the edge of the negative electrode 21, consequently welding the plate 3 and the negative electrode edge to each other completely.
In the step of welding the positive electrode current collector plate 30 to the edge of the positive electrode 23 with a laser beam, the aluminum forming the collecting plate 30 is low in laser beam reflectivity, so that the energy of the laser beam can be fully given to the junction of the collector plate 30 and the edge of the positive electrode 23, consequently welding the plate 30 and the positive electrode edge to each other completely.
In the assembling step, the positive electrode current collector plate 30 and the negative electrode current collector plate 3 are electrically connected to the positive terminal portion and the negative terminal portion, respectively.
This sufficiently lowers the electric resistance of the conductors extending from the electrode unit 2 to the terminal portions to achieve a high current collecting efficiency.
The nonaqueous electrolyte secondary cell and the process for fabricating the cell according to the invention give the negative electrode current collector plate improved weldability to the electrode unit, whereby a high current collecting efficiency can be attained as described above.
Construction for Fulfilling Second Object
Another nonaqueous electrolyte secondary cell comprises an electrode unit 7 encased in a battery can 1 and comprising as superposed in layers a positive electrode 71, a negative electrode 73 and a separator 72 interposed between the electrodes and impregnated with a nonaqueous electrolyte, each of the positive electrode 71 and the negative electrode 73 being formed by coating a striplike current collector with an active material. The cell is adapted to deliver electric power generated by the electrode unit 7 to the outside via a pair of electrode terminals.
The current collector of the positive electrode 71 or the negative electrode 73 has a projecting edge 78 at at least one of opposite ends of the electrode unit 7, and a current collector plate 8 is joined to the edge 78 and has a plurality of protrusions 82 formed on a surface thereof opposed to the current collector edge 78. Each of the protrusions is shaped to have a circular-arc section or polygonal (e.g., trapezoidal) section with at least four corners, the collector plate 8 being welded to the current collector edge 78 with the protrusions 82 forced therein and being connected to one of the electrode terminals.
The present invention further provides a process for fabricating a nonaqueous electrolyte secondary cell which process has the steps of:
preparing an electrode unit 7 wherein an edge 78 of current collector of each of a positive electrode 71 and a negative electrode 73 is positioned as projected outward beyond an edge of a separator 72 by laying the positive electrode 71 and the negative electrode 73 over the separator 72 as displaced from the separator widthwise thereof and rolling up the resulting assembly into a spiral form,
preparing current collector plates 8 each by forming in a flat platelike body 81 having electric conductivity a plurality of protrusions 82 each shaped to have a circular-arc section or polygonal section having at least four corners,
welding the collector plates 8 respectively to the projecting current collector edges 78 at the respective ends of the electrode unit 7 by placing each collector plate 8 over the current collector edge 78 in pressing contact therewith and irradiating each protrusion 82 of the collector plate 8 with a laser beam or electron beam, with the protrusion 82 forced into the current collector edge 78, and
placing the electrode unit 7 having the collector plates 8 welded thereto into a battery can 1 and connecting the collector plates 8 to respective electrode terminals.
With the nonaqueous electrolyte secondary cell and the fabrication process thereof according to the invention described, the current collector plate 8 is pressed against the current collector edge 78 of the electrode unit 7, whereby each protrusion 82 of the collector plate 8 is forced or wedged into the current collector edge 78, forming a joint face in the current collector edge 78 in conformity with the contour of the protrusion 82, for example, a cylindrical joint face. The joint face has a larger area than is formed by a protrusion which is V-shaped in section.
Accordingly, when the collector plate 8 is welded to the current collector edge 78 by irradiating the junction of each protrusion 82 and the current collector edge 78 with a laser beam or electron beam, the plate 8 is joined to the current collector edge 78 over a large area of contact. This results in diminished contact resistance and a higher current collecting efficiency.
The junction of the collector plate protrusion 82 and the current collector edge 78 will be positioned at 90xc2x0 or approximately at this angle with the direction of projection of the beam at the midportion of the junction, so that the laser beam or electron beam acts effectively for welding the junction, consequently affording a high weld strength due to the large area of the junction.
Stated more specifically, the current collector plate 8 comprises a flat platelike body 81 formed with the protrusions 82 and one or a plurality of liquid inlets 83, and the opening area provided by the liquid inlets 83 is at least 15% of the flat area of the body. When the electrolyte is placed into the cell 1 can in the step of assembling the cell, the electrolyte flows through the liquid inlets 83 in the current collector plate 8 of this structure and is fed to the electrode unit 7. This shortens the time required to impregnate the separator 72, positive electrode 71 and negative electrode 73 with the electrolyte. If the opening ratio provided by the liquid inlets 83 is smaller than 15%, the electrolyte encounters difficulty in passing through the collector plate 8 and therefore requires a prolonged period of time for impregnation. However, if the opening ratio given by the liquid inlets 83 is in excess of 90%, the current path becomes greatly constricted, increasing the electric resistance of the collector plate 8 and leading to a lower current collecting efficiency. Accordingly, it is desirable that the opening ratio given by the liquid inlets 83 be in the range of 15% to 90%.
Alternatively, the current collector plate 8 comprises a flat platelike body 81 formed with the protrusions 82 and integrally provided with a striplike lead portion 85, the lead portion 85 having an outer end connected to the electrode terminal. The lead portion 85 of this structure is easily connectable to the electrode terminal, further serving to diminish the electric resistance between the electrode unit 7 and the electrode terminal.
A current collector plate 100 of another structure comprises a flat platelike body 101 provided at an outer peripheral portion thereof with a current collector pressing portion 106 positioned in the vicinity of each protrusion 102 for pressing an end portion of the current collector 77 of the electrode unit 7 inwardly of the electrode unit 7. With this structure, the end portion of the current collector 77 is deflected inwardly of the electrode unit 7 by being pressed by the current collector pressing portion 106, whereby the position of contact of the current collector end with the protrusion 102 of the collector plate 100 is shifted also inwardly of the electrode unit 7. Accordingly, when the collector plate protrusion 102 is to be welded to the end portion of the current collector 77, the laser beam or electron beam need not be projected onto the radial outer end of the protrusion but the protrusion needs only to be irradiated up to a position slightly inwardly of its outer end, i.e., up to the position where the deflected portion of the current collector 77 is in contact with the protrusion. This eliminates the likelihood that the beam will be projected outside beyond the outer periphery of the collector plate 100, consequently precluding the current collector 77 or separator 72 from melting by being directly irradiated with the beam.
The pressing face of the current collector pressing portion 106 for the current collector 77 and the surface of the platelike body 101 of the collector plate 100 make an angle in the range of at least 30xc2x0 to not greater than 45xc2x0. When the angle is limited to this range, the outer end of the current collector 77 can be effectively deflected inwardly of the electrode unit 7.
According to the process of the invention described for fabricating nonaqueous electrolyte secondary cells, it is desirable that the protrusions 82 of the collector plate 8 have a width at least 0.8 times the diameter of the spot of the laser beam or electron beam. For example, when the protrusion 82 of the collector plate 8 has a semicircular form in section, it is desired that the diameter of the semicircle be at least 0.8 times the spot diameter of the laser beam or electron beam. Further when the collector plate protrusion 82 has a trapezoidal form in section, it is desired that the width of the upper side (short side) of the trapezoid be at least 0.8 times the spot diameter of the laser beam or electron beam. This enables the laser beam or electron beam to give energy concentrically on the junction of the collector plate protrusion 82 and the current collector edge 78, fully melting the portions to be joined and giving a large joint area and high weld strength.
The distance the protrusion 82 of the collector plate 8 projects is preferably at least 0.5 mm to not greater than 3 mm. If the distance of projection of the protrusion 82 is smaller than 0.5 mm, it is impossible to force the protrusion 82 into all turns of the current collector at the edge 78 in the case where the edge portions 78 of turns of the current collector of the electrode unit 7 are not positioned uniformly in a plane, consequently failing to afford sufficient weld strength. Further when the distance of projection of the protrusion 82 is in excess of 3 mm, the effect to improve the weld strength will level off, while a greater dead space is created in the interior of the battery can 1 to entail a lower energy density relative to the volume.
The thickness of the current collector plate 8 is preferably at least 0.1 mm to not greater than 2 mm. If the thickness is smaller than 0.1 mm, the collector plate 8 has increased electric resistance to exhibit a lower current collecting efficiency. Further if the thickness is greater than 2 mm, the effect to improve the current collecting efficiency levels off, while the lead portion 85 formed integrally with the plate 8 will not be workable without a problem.
Further it is desired that the wall thickness of the protrusion 82 of the current collector plate 8 be smaller than the thickness of the flat platelike body 81. The flat portion then has a greater thickness, ensuring a satisfactory current collecting efficiency without impairment, while the portion to be irradiated with a beam has a small thickness and therefore permits welding with low energy.
Usable as the material for the current collector plate 8 is Cu, Al, Ni, SUS, Ti or an alloy of such metals. Use of these materials provides cells which are excellent in corrosion resistance to nonaqueous electrolytes and in conductivity.
According to the present invention providing nonaqueous electrolyte secondary cells and processes for fabricating such cells, the current collector plate can be joined to the current collector edge over a large contact area even if the current collector forming the electrode unit has a very small thickness as described above, hence high productivity.